The invention relates to an operating method of an electric-energy generating, -providing and -consuming system of a motor vehicle, consisting of a fuel cell system having at least one hydrogen (air) oxygen fuel cell, an accumulator and an electric motor acting, in particular, as a vehicle driving motor. By the interposition of switching elements, these components are electrically coupled in such a manner that the fuel cell system can supply the electric motor as well as the accumulator with electric energy, and the accumulator can supply the electric motor with electric energy.
Conventional electric motor vehicle systems having a fuel cell system for generating electric energy, an accumulator for storing energy and an electric motor, which is fed from these above-mentioned elements, for driving the motor vehicle, have an electric voltage converter (in the form of a dc/dc converter), connected on the output side of the fuel cell system. The voltage converter transforms the electric voltage level of the fuel cell system to the normal voltage level of the high-performance onboard power supply system of the vehicle, i.e. to the voltage level of the accumulator as well as of the electric motor, which may, for example, be in the order of 150 V. As a function of the further development or of the operating principle of the electric motor acting as the driving motor, an inverter can be connected to the input side of the electric motor, which inverter transforms the direct current of the fuel cell system as well as of the accumulator to alternating current, in the process, however, changing the voltage level only insignificantly.
Since an electric voltage converter mentioned in the preceding section has to be designed for the maximal output power of the fuel cell system or at least for the maximal consuming power of the electric vehicle driving motor, it is a high-expenditure component which requires a large installation space, has a high weight and should not be necessary.
It is an object of the present invention to provide a measure for eliminating an electric voltage converter for the operation of an electric energy system of a motor vehicle.
The solution of this task is characterized in that, by a design of the system such that the accumulator can essentially be charged completely by the fuel cell system alone, as well as by the electric motor operating as a generator alone, without the implementation of an electric voltage conversion and therefore without providing an electric voltage converter, as a function of the level of the electric voltage provided by the fuel cell system and of the electric voltage level offered by the accumulator and the electric power demanded by the electric vehicle driving motor, a first switching element in the electric connection between the fuel cell system and a node point electrically connected with the accumulator and the electric motor as well as a second switching element in the connection between the accumulator and the above-mentioned node point are opened or closed as needed, i.e. are moved into a position not permitting a flow of current or into a position permitting the flow of current. The “concrete” meaning of “as needed” is that the electric power requirement of the electric motor is met primarily from the fuel cell system and, in an auxiliary fashion, additionally from the accumulator, and that, if the electric power made available from the fuel cell system, recognizable by its voltage level, exceeds the electric power demanded by the electric motor, the excess power of the fuel cell system resulting from the difference between available amount and the demand will be fed to the accumulator for as long as the maximally permissible charging current of the accumulator is not exceeded and the accumulator still has a residual storage capacity at least in the amount of the kinetic energy of the motor vehicle that can be fed to the accumulator in the current driving state of the vehicle by recuperation by way of the electric motor operating as a generator.
By means of the characteristics according to the invention, an operating strategy of a system of the above-mentioned type, which can be implemented by an electronic control unit, or a switching strategy for the above-mentioned switching elements, is provided, which makes it possible to operate the fuel cell system with high efficiency without a voltage conversion and therefore also without the use of an electric voltage converter and to simultaneously also minimize stressing of the accumulator by conditions which reduce its service life. For this purpose, the fuel cell system is designed such that its electric nominal voltage, i.e. the height of the providable voltage level is greater than the electric (nominal) voltage of the completely charged accumulator. The latter can thereby essentially be charged completely, i.e. fully, by the fuel cell system as well as the electric motor, when the electric motor is operated as a generator. In this case, the electric motor acting as a vehicle driving motor will then be operated as a generator and therefore be driven by the motor vehicle, when this motor vehicle is braked, i.e. its kinetic energy is to be reduced. The kinetic energy of the motor vehicle can thereby be recuperated (reduced by the conversion losses) and can be intermediately stored in the accumulator.
By this operating strategy, the basic supply of the electric motor is covered by the fuel cell system, and only the peak demand of the electric motor is additionally covered from the accumulator. An electronic control unit can recognize, by means of the voltage level provided by the fuel cell system, whether such a peak demand exists, and then additionally connect the accumulator for supplying the electric motor. It is thereby achieved that the accumulator experiences no unnecessary charging and discharging cycles, which would significantly reduce its service life and efficiency
The accumulator is recharged primarily by recuperation, which is why, according to the invention, an additional charging of the accumulator by the fuel cell system, if the latter is, recognizably by an electronic control unit, capable of doing so on the basis of a correspondingly low power demand by the electric motor, will be implemented only until the accumulator still has a residual storage capacity, which can accommodate at least the kinetic energy existing in the current driving state of the motor vehicle in the form of recuperated electric energy.
In addition, in the case of the charging of the accumulator by the fuel cell system, it is taken into account that the electric charging current at the accumulator should be lower than the charging current maximally permissible for this accumulator. If the last-mentioned criterion cannot be met or if the accumulator is already sufficiently charged, which an electronic control unit can determine by way of its voltage level, the electric connection between the fuel cell system and the accumulator will be interrupted by the suitable switching element. If, in this case, the electric motor also demands no power, the fuel cell system will automatically move to idle operation because of a lack of power consumption; when power is demanded by the electric motor, the fuel cell system will then automatically supply only as much power as is demanded.
The above-mentioned electronic control unit, which executes the operating method according to the invention, preferably controls the above-mentioned switching elements such that the charging state of the accumulator is kept in a range favorable with respect to its service life and efficiency during charging and discharging, which range may preferably be in the order of from 30% to 80%. Naturally, the electronic control unit, when activating the above-mentioned switching elements, will further take into account that no electric current originating from the accumulator or from the electric motor operating as a generator, can arrive in the fuel cell system. For this purpose, the voltage level, which is currently present at the node mentioned above, is compared with the voltage level provided by the fuel cell system. If the latter is not higher than the voltage level at the above-mentioned node, the electric connection between the fuel cell system and the above-mentioned node has to be interrupted or will be interrupted by opening or keeping open the switching element provided in this connection.
The possibilities contained in this operating strategy described so far (and in the following description of the figures with the amendments) can be considerably expanded by a targeted influencing of the voltage level provided by the fuel cell system. If this provided voltage level is significantly higher than the voltage level that can be processed by the electric motor together with the accumulator while taking into account the conditions which were mentioned above and limit the charging of the accumulator by the fuel cell system to certain cases, the voltage level of the fuel cell system can be adapted to the demand by the modulation of the oxygen content, specifically the lowering of the oxygen content, on the cathode side of the fuel cell(s). The efficiency of the fuel cell system is thereby not significantly impaired. The corresponding situation applies when the electric voltage level provided by the fuel cell system is significantly lower than the voltage level demanded by the electric motor or by the accumulator while taking into account the above mentioned conditions which limit the charging of the accumulator by the fuel cell system to certain cases. Then, the voltage level of the fuel cell system can be adapted to the demand by modulation and here by raising of the oxygen content on the cathode side of the fuel cell(s).
The oxygen content can correspondingly be modulated on the cathode side of the fuel cell(s) of the fuel cell system by suitable measures at the air flow fed to the fuel cell system or to the cathode sides of the fuel cells. When this air flow is mixed with a portion of the exhaust gas flow of the fuel cell system having a considerably reduced oxygen content, the oxygen content of the fed air flow will naturally drop. Therefore, for the lowering of the voltage level of the fuel cell system, as required, the rate of fuel cell exhaust gas returned to the cathode side of the fuel cell(s) can be changed. However, the air throughput on the cathode side of the fuel cell(s) can also be changed directly, but such a change has limits, in which case the fuel cell system could be damaged when there is a falling-below those limits. An increase of the oxygen content of the air flow supplied to the cathode side of the fuel cell(s) for raising the voltage level of the fuel cell system, as required, can take place by adding oxygen from a suitable short-term storage device for oxygen, for example, in the form of a zeolite, which had previously been filled with oxygen from fed air. As an alternative or in addition, by use of a molecular sieve or the like, the nitrogen fraction in the air flow supplied to the fuel cell system can be reduced, whereby the oxygen fraction in this air flow will necessarily be increased.
Finally, the method continuously monitors the electric voltage of all or each individual fuel cell(s) of the fuel cell system with respect to observing the limit values permissible for the latter and to prevent an exceeding or falling-below these limit values by changing the fed fuel quantity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.